We propose to image regional variations in the absorption properties of biologic tissue in the near infrared and microwave regions of the electromagnetic spectrum using photoacoustic ultrasound (PAUS) imaging principles developed recently in our laboratory. Research to date has demonstrated that it should be possible to image low contrast structures in a 5.6 cm compressed breast using safe levels of infrared radiation (lambda = 1.064 micromoles). We will extend our work in the infrared to a point where we can image excised breast tissue in vitro. We also plan to extend our investigations to 433 MHz microwave radiation where image contrast and tissue penetration are expected to be significantly better than in the infrared. During the three-year research period we will: i. Simulate the ability of PAUS to image optical energy deposition patterns within the human breast at 1.064 micromoles, based on a set of photoacoustic measurements made over the surface of a cylinder. ii. Develop and test PAUS instrumentation to perform preliminary experiments at 433 MHz. iii. Develop and test PAUS instrumentation for imaging human tissues in vitro at 1.064 micromoles. iv. Develop and test algorithms for demodulating the smoothly varying, depth-dependent excitation energy from PAUS images obtained at 1.064 micromoles. v. Develop and test PAUS instrumentation for imaging human tissues in vitro at 433 MHz. vi. Use PAUS instrumentation to quantify image contrast and contrast patterns of excised, human breast tissue in vitro at both 1.064 micromoles and 433 MHz.